1. Field of the Invention
This invention relates to a process for catalytic vapor phase oxidation and a reactor used therefor and, in particular, to a process for catalytically oxidizing hydrocarbons in vapor phase using a fixed bed multi-tubular heat exchange type reactor. More specifically, it concerns a structure of a multi-tubular heat exchange type reactor which can keep a catalyst used therein at optimum reaction conditions and restrict the generation of hot spots (abnormal local heating in catalyst layers), upon exothermic catalytic vapor phase oxidation of hydrocarbons, and a method of using such a reactor.
2. Discussion of the Relevant Art
Catalytic vapor phase oxidations are generally highly exothermic and it is thus very important to control the reaction temperature within a certain range and restrict the generation of hot spots in the reaction zones, which imposes great efforts on those skilled in the art. No satisfactory control of the catalytic reaction temperature with the catalysts can be attained only with the uniform circulation of heat transfer medium in a reactor and hot spots appear frequently to result in excess oxidation locally in the reactor, particularly where the oxidation reaction has to be proceeded sequentially to convert starting materials into end products such as in the oxidation of naphthalene or o-xylene into phthalic anhydride, oxidation of benzene, butylene or butadiene into maleic anhydride, oxidation of propylene into acrolein or acrylic acid, oxidation of ethylene into ethylene oxide, ammoxidation of propylene into acrylonitrile, ammoxidation of aromatic hydrocarbons such as toluene and xylene into aromatic nitriles sch as benzonitrile, phthalonitrile and the like. As the result undesired combustion reaction is increased to lower the yield of the aimed products. In addition, since the catalysts are always exposed locally to high temperature by the presence of the hot spots, the life of the catalysts is decreased in that portion to result in disadvantages.
Various counter measures have been employed in order to overcome the foregoing disadvantages in the vapor phase oxidation. As one of the most popular methods, the diameter of catalyst filled tube is decreased in order to increase the heat transfer rate per unit volume of the catalyst. This method is, however, defective in that the number of the filled tube is increased and it increases the fabrication cost of the reactor, as well as takes much time for the charging and discharging of the catalyst.
In other effective methods proposed so far, catalyst layer is diluted with an inert substance or, as disclosed in Japanese Published Unexamined patent application No. 85485/1973, generation of hot spots is restricted by the insertion of cylindrical material containing closed cavity in the center at the cross section of a reaction tube filled with the catalyst entirely or partially in the axial direction of it thereby providing a space in which no catalyst is present and no reaction mixture passes through. This method is, however, defective in that the cost is inevitably increased by so much as the substantially inert material is contained. It has a further defect that recovery of useful metal components from the catalyst removed from the reactor after the degradation of the catalytic activity is very laborious to lower the recovery efficiency.
A further effective method suppresses the temperature rise in the hot spots by gradually increasing the activity of the catalyst from the inlet to the outlet in the reaction tube. This method, however, requires at least two types of catalysts of different catalytic activities, and no optimum reaction temperatures can be selected for respective catalysts charged in each of the layers. Moreover, if these catalysts show different degrees of aging changes in their catalytic activities, control and keeping of the optimum reaction temperatures are further difficult to inevitably lower the over all yield for the desired products.
A still further effective method is proposed as disclosed in U.S. Pat. No. 3,147,084 and in German Laid Open Pat. Publication No. 2,513,405 wherein a shell of a multi-tubular heat exchange type reactor is entirely partitioned with a shield plate into two heat transfer medium feed zones and reaction is carried out while circulating heat transfer medium at different temperatures in each of the zones. It is, however, very difficult in this method to insert as many as several thousands of reaction tubes into a perforated plate used as the shield plate in the reactor, and those portions between the perforated plate and the reaction tubes that are contacted by the heat expansion of the tubes are abraded by the pulsation of the heat transfer medium to cause corrosion and destruction unless the reaction tubes and the perforated plate are secured to each other by welding or expanding the diameter of the reaction tubes. The above securing fabrication however requires troublesome works such as accurate perforation, welding and diameter expansion over several thousands of portions.
It is, accordingly, an object of this invention to provide an improved process of catalytic vapor phase oxidation and a reactor used therefor.
It is another object of this invention to provide a process of catalytic vapor phase oxidation in which catalyst is kept at its optimum reaction conditions and an apparatus used therefor.
It is a further object of this invention to provide a structure of a multi-tubular heat exchange type reactor capable of restricting the generation of hot spots and a method of using such a reactor.
These objects of this invention can be attained by the catalytic vapor phase oxidation process which comprises using a fixed-bed shell and tube heat exchange type reactor in which a bundle of multiplicity of tubes filled with at least one type of oxidation catalyst is disposed in a shell and these tubes are passed through the apertures perforation in at least one perforated shield plate to partition the shell into at least two heat transfer medium feed zones in such a way that each of the tubes passed through the perforated shield plate is not in direct contaction with the shield plate but the outer surface of the tubes and the inner surface of the apertures are spaced apart with a distance between 0.2-5 mm, feeding feed gas to the tubes of the reactor, and conducting the exothermic catalytic vapor phase oxidation while controlling the temperatures of the heat transfer medium in each of the zones partitioned by the shield plate so that the temperature difference therebetween is kept within the range between 0.degree.-100.degree. C.
In order to obtain high yield an improved reactor for varying reaction temperatures corresponding to reaction stages has hitherto been proposed (Japanese Published Unexamined patent application No. 80473/1973.) However, settlement of the temperature described in the patent application No. 80473/1973 is to carry out the reaction smoothly by providing a controller in a circulation mechanism of the heat transfer medium, so it is difficult to obtain relatively isolated reaction temperature zones as in the present invention. In the above process it is rather proposed to provide reaction zones wherein the heat transfer mediums are completely separated from each other as a means for obtaining such shielded reaction temperature zones. Therefore, it is clear that the present invention relates to a simple and economical reactor having a novel construction.
The reactor specified hereinbefore for use in the process of the present invention has an advantage in that the structure is simple to make, has reduced cost and, in addition, as detailed hereinafter, it is not distorted by heat. The use of this reactor in catalytic vapor phase oxidation enables one to control the temperature of the heat transfer medium in the catalyst layer region where the exotherm is most significant to a lower level than the temperature for the heat transfer medium in the other region thereby restricting the exotherm in the hot spots. This enables one to increase the conversion rate of the feed gas to be oxidized in the succeeding zone substantially to 100% and thus permits most effective utilization of the catalyst. Particularly, the use of the above reactor according to this invention in the catalytic vapor phase reaction where the reaction is proceeded sequentially, restricts useless combustion caused by the over oxidation in the hot spots to ensure improved yield in the desired products and enables to increase the concentration of the starting material as compared with that in the conventional catalytic vapor phase reaction. The reaction where catalysts and reaction temperatures are different in each of the reaction steps and, hence, two or more reactors have been required so far can be conducted with only one reactor by the process according to this invention. Moreover, this invention provides a further merit that the catalyst life is prolonged astonishingly.